Laser ultrasonic measurement systems are frequently used for testing of various components and complex structures. These systems offer advantages over non-laser type systems (e.g., piezoelectric transducer-based systems). Laser ultrasonic systems are typically non-contact systems that test a structure by measuring ultrasonic waves induced in a structure. Typically, a short laser pulse is directed to a structure causing thermal expansion of the structure, which generates ultrasonic waves. Laser ultrasonic systems are well suited for many industrial applications such as measurement of steel at high temperature, measurement of paint thickness, and non-destructive testing of complex composite structures.
One drawback of existing confocal Fabry-Perot (CFP) interferometers for laser ultrasonic measurement systems is the difficulty of controlling the interferometer for optimal performance. In general, for optimal performance, the CFP cavity length must be continuously adjusted to compensate for drift in the laser frequency and changes in the length due to thermal expansion. In some configurations it is also possible to adjust the laser frequency to match a fixed-length cavity to compensate for cavity length changes and laser frequency drift.